The present invention relates to an inter-vehicle distance measurement apparatus, which carries out simple processing to confirm the presence of a vehicle and to reliably determine the inter-vehicle distance to a preceding or following vehicle, and detects the dangerous approach of the following vehicle based on the inter-vehicle distance to the following vehicle and the speed of its own vehicle to alert the following vehicle by, e.g. flashing the brake light.
In the apparatus, m .times. n distance information is obtained by using a light-receiving device with (m) optical sensor arrays disposed such that the longitudinal direction is approximately aligned with the vertical direction, each of the (m) sensor arrays having (n) measurement windows spaced at a pixel interval. The apparatus detects from the m .times. n distance information, a white line on a road surface to limit the area in which a vehicle may be present, and confirms the presence of the vehicle.
In the drawings filed in the application, the same reference numerals designate the same or equivalent components.
In the known inter-vehicle distance measurement apparatuses, images formed by two lateral optical systems are electrically compared to measure the distance based on the principle of triangulation.
FIG. 24 shows a conventional inter-vehicle distance measurement apparatus. In this figure, image forming lenses 1 and 2 are disposed at an optical axis interval B. Optical sensor arrays 3A and 4A are, for example, CCD linear sensor arrays and are each located at a focal length (f) from the image forming lens 1 or 2. The optical sensor arrays 3A and 4A convert an image of a target 13' formed by the image forming lenses 1 and 2 into image signals 30A and 40A and output to a signal processing section 5.
The signal processing section 5 comprises amplifiers 51 and 52; A/D converters 53 and 54; and a storage or memory device 55. The image signals 30A and 40A from the optical sensor arrays 3A and 4A are amplified by the amplifiers 51 and 52, converted into digital data by the A/D converters 53 and 54, and then outputted to the storage device 55 as image data 31A and 41A.
A distance measurement circuit 6 disposed on the output side of the signal processing section 5 comprises a microcomputer, which compares the lateral image data 31A and 41A stored in the storage device 55 to calculate the distance to the target 13', and outputs a result as a distance signal 9.
Next, a principle of the calculation of the distances is described with reference to FIG. 25. A horizontal axis X and a vertical axis Y are set by using a midpoint of the image forming lenses 1 and 2 as an origin O, and the coordinates of the image forming positions L.sub.1 and R.sub.1 are referred to as (-a.sub.L1 -B/2, -f) and (a.sub.R1 +B/2, -f), respectively, wherein a.sub.L1 and a.sub.R1 are distances on the optical sensor arrays 3A and 4A as illustrated.
The coordinates of the midpoint O.sub.L of the image forming lens 1 are (-B/2, 0), and the coordinates of the midpoint O.sub.R of the image forming lens 2 are (B/2, 0). If the coordinates of a point M in the target 13' are referred to as (x, y), the coordinates of the intersection N of a vertical line extending from the point M to the X-axis are (x, 0), the coordinates of position L.sub.O of a vertical line extending from a point O.sub.L to the optical sensor array 3A are (-B/2, -f), and the coordinates of a position R.sub.O of a vertical line extending from a point O.sub.R to the optical sensor array 4A are (B/2, -f). In this case, since .DELTA.MO.sub.L N is similar to .DELTA.O.sub.L L.sub.1 L.sub.O, and .DELTA.MO.sub.R N is similar to .DELTA.O.sub.R R.sub.1 R.sub.O, the following equations (1) and (2) are satisfied: EQU (x+B/2)f=(a.sub.L1 +B/2-B/2)y Equation 1 EQU (-x+B/2)f=(a.sub.R1 +B/2-B/2)y Equation 2
The following Equation (3) can be obtained from Equations (1) and (2). EQU y=B.multidot.f/(a.sub.L1 +a.sub.R1) Equation 3
By Equation (3), the distance (y) to the target 13' can be obtained if the distances a.sub.L1 and a.sub.R1 for the image forming positions L1 and R1 are known.
Next, the operation of the distance detection circuit 6 is described in detail. The distance detection circuit 6 compares lateral or right and left image data 3AT and 4AR such as that shown by the solid lines in FIG. 26 with respect to separately set measurement window parts, and if they do not match, for example, the image data 3AL on the left is sequentially shifted to the right, while the image data 4AR on the right is sequentially shifted to the left, as shown by the broken lines in the figure. The shift distance is then detected when the image data on the right and left match.
An evaluation function is used to determine the degree of coincidence between the right and left image data 3AL and 4AR. The evaluation function is obtained by adding the absolute values of the differences between pixel data for all pixels (in this example, CCD elements) located at corresponding coordinates (addresses) in measurement windows located Within the optical sensor arrays 3A and 4A on the right and left. The value of the evaluation function is examined while the right and left measurement windows are sequentially shifted, that is, the left measurement window is shifted to the left (image data 3AL on the left is equivalently shifted to the right), whereas the right measurement window is shifted to the right (image data 4AR on the right is equivalently shifted to the left). The data on the right and left is determined to match when the function has a minimum value.
The distances a.sub.L1 and a.sub.R1 for the right and left image forming positions L.sub.1 and R.sub.1 described above are equal to these shifted distances, so the distance detection circuit 6 can calculate the distance (y) to the target 13' based on the shifted distances a.sub.L1 and a.sub.R1 through the use of Equation (3) shown above.
The conventional principle of the measurement of a plurality of points in the longitudinal direction of the optical sensor arrays is described with reference to FIG. 27. The distance measurement apparatus in this case has the same structure as in FIG. 24 except that each sensor array is partitioned into a plurality of regions, i.e. measurement windows. FIG. 27 shows a case in which the optical sensor arrays are partitioned into three regions (a), (b) and (c).
Targets O.sub.1, O.sub.2 and O.sub.3 for which the distance is to be measured are located at the distances L.sub.1, L.sub.2 and L.sub.3, respectively, from the distance measurement apparatus, in the three directions shown by the alternating long and short lines, that is, the direction of the center line and the directions with an angle .alpha. on both sides of the center line. The regions (a), (b) and (c) which form pairs between the optical sensor arrays 3A and 4A correspond to the targets O.sub.1, O.sub.2 and O.sub.3, respectively.
In other words, an image of the target O.sub.1 located left at the angle .alpha. relative to the center line is simultaneously formed in the pair of regions (a) of the optical sensor arrays 3A and 4A, an image of the target O.sub.2 located at the center line is simultaneously formed in the two regions (b), and an image of the target O.sub.3 located right at the angle .alpha. relative to the center line is simultaneously formed in the two regions (c). The distances L.sub.1, L.sub.2 and L.sub.3 to the targets O.sub.1, O.sub.2 and O.sub.3 can be expressed by the following Equations (4) to (6): EQU L.sub.1 =B.multidot.f/(U.sub.21 -U.sub.11) Equation 4 EQU L.sub.2 =B.multidot.f/(U.sub.22 +U.sub.12) Equation 5 EQU L.sub.3 =B.multidot.f/(U.sub.13 -U.sub.23) Equation 6
Distances B, (f), U.sub.11, U.sub.12, U.sub.13, U.sub.21, U.sub.22 and U.sub.23 in these equations are as shown in FIG. 27.
Since each shift distance (U.sub.21, U.sub.11, U.sub.22, U.sub.12, U.sub.13, or U.sub.23) can be determined by the distance detection circuit 6 based on the image data from the optical sensor arrays 3A and 4A, the distances L.sub.1, L.sub.2 and L.sub.3 can be determined by Equations (4) to (6).
In this manner, the conventional method measures the distance for each of the plurality of the measurement windows set within the optical sensor arrays to extract the location of a vehicle by using the following techniques.
For example, according to the technique disclosed in Japanese Patent Application Laid-Open No. 8-210848, which is the applicant's previous application (hereinafter referred to as "first previous application"), (n) measurement points are measured in the lines (hereinafter referred to as "sensor lines") in each of (m) sensor arrays to which (n) measurement windows are allocated, and the frequency distribution of the distances within the m .times. n distance matrix is determined. The average movement of a distance block region corresponding to a size of a vehicle is then determined within the distance matrix, and an object assumed to be a vehicle is identified. Finally, its distance is extracted.
In addition, according to the technique disclosed in Japanese Patent Application Laid-Open No. 7-280563, which is another applicant's application (hereinafter referred to as the "second previous application"), a white line is distinguished based on the distance to and the width of an image assumed to be the white line on the surface of a road on which the vehicle is traveling. Then, based on the white line, the distance measurement range is determined to detect the distance to a vehicle within the range.
In addition, the well-known inter-vehicle distance measurement apparatus includes a buzzer to alert the driver if the inter-vehicle distance determined in the above manner is smaller than a safe inter-vehicle distance calculated based on tale speed of the vehicle and the relative speed, and laser radar distance measurement apparatuses have been practically used for some large trucks.
FIG. 23 shows how the safe inter-vehicle distance is determined based on the speed of its own vehicle and the relative speed. This figure shows the relationship between the speed of the vehicle and the safe inter-vehicle distance at a particular relative speed. When the speed of its own vehicle is referred to as V.sub.2 ; the speed of the preceding car is referred to as V.sub.1, the deceleration of the preceding car is referred to as .alpha..sub.1 ; the deceleration of its own vehicle is referred to as .alpha..sub.2 ; the delay time (free running time) from the start of the deceleration of the preceding vehicle until the start of the deceleration of the original vehicle is referred to as T.sub.delay ; and the margin of the distance (inter-vehicle distance when both vehicles are completely stopped) is referred to as D.sub.1 ; the relative speed can be expressed as V.sub.2 -V.sub.1 and the safe inter-vehicle distance D.sub.safe can be expressed by the following Equation (7): ##EQU1##
In this equation, V.sub.2 is obtained from a vehicle speed signal from a vehicle speed sensor installed in the own vehicle, and V.sub.1 is determined based on the inter-vehicle distance and the vehicle speed signal. D.sub.1 is fixed as a certain constant. In addition, T.sub.delay, .alpha..sub.1, and .alpha..sub.2 are constants but vary depending on the driver's driving skills and the conditions of the road surface. Thus, these values must be determined while considering these factors.
The conventional inter-vehicle distance measurement apparatus has the following problems:
FIG. 28 shows an example of an inconvenient relationship between an image of a vehicle and measurement windows within the sensor lines in a plurality of optical sensor arrays installed parallel approximately in a vertical direction as the longitudinal direction. According to the distance measurement apparatus according to the first previous application (Japanese Patent Application Laid-Open No. 8-210848), the measurement windows are provided at the circles as a center in each line in FIG. 28 to measure the distances. The size of an image of the vehicle formed on a measurement visual field decreases as the inter-vehicle distance increases. As shown in the figure, the image of the vehicle may be offset from the center of the measurement window and present only in a part of it, depending on the situation, to make measurement of the distance to the vehicle inaccurate or difficult.
To prevent such an inconvenience, for example, the following methods can be used:
A first method is to increase the width of the measurement window so that an image of the vehicle formed in a corner of the window can be used to determine the distance to the vehicle despite the absence of the vehicle in the center of the window. This method, however, causes an image of the vehicle and images of the background and road surfaces to be shown in the same window, resulting in a larger error in the measurement of the distance due to the mixture of far and near objects.
A second method is to increase the number of the measurement windows instead of increasing the width in order to guarantee that a certain number of windows is used to detect the distance to the vehicle. This method, however, increases the amount of distance data to be obtained to thereby require a long time to extract frequency distributions or determine the average movement. Consequently, it requires a CPU with a larger capacity to reduce the processing time, resulting in high costs.
Thus, a method that prevents errors caused by the mixture of far and near objects and that can extract the location of a vehicle by using simple processing is required.
A possible method for reducing the processing time required to extract the location of a vehicle is to use the technique disclosed in the second previous application (Japanese Patent Application Laid-Open No. 7-280563) to detect a white line on a road surface in order to determine the distance measurement range based on that white line, thereby determining the processing range within which the location of a vehicle is to be extracted. This method, however, can reduce the time required to extract the location of a vehicle, but a longer time is required to detect the white line due to the use of the image data, thereby preventing the required reduction of the sampling frequency. Thus, a more simple processing method that eliminates the need for image data in the detection of the white line is required.
Furthermore, according to the conventional method, the following inconvenience occurs when the inter-vehicle distance determined in the above manner is used to output an alarm. In the conventional method in which an alarm is outputted when the inter-vehicle distance apparatus described above has measured the distance to the preceding vehicle, an alarm is frequently outputted when the inter-vehicle distance has become smaller than the safe inter-vehicle distance, which bothers the driver and makes the driver to be accustomed.
To prevent this, the alarm determination threshold in Equation (7) must be determined appropriately to clearly distinguish the dangerous condition from the safe condition in order to make the alarm more reliable.
The constants T.sub.delay, .alpha..sub.1 and .alpha..sub.2 in Equation (7), however, vary depending on the various parameters, such as driver's driving skills, habits and physical condition; conditions and slopes of the road surfaces; and abrasion of tires. These factors can not be measured easily, and the alarm is likely to be outputted even in a safe situation. Thus, an inter-vehicle distance measurement apparatus that is acceptable to the driver due to its ability to prevent the driver from being bothered and can consistently alert the driver to potential danger is required.
It is thus an object of the invention to solve these problems of the conventional techniques and to provide an inter-vehicle distance measurement apparatus that uses light receiving devices with optical sensor arrays to stably, accurately and simply determine the distance to the preceding or following vehicle and that prevents the driver from being bothered while sufficiently alerting the driver to potential danger.